1. Field of the Invention (Technical Field)
The present invention relates to wavefront and surface metrology employing a Hartmann sensors.
2. Background Art
For a number of years, there has been growing interest In the application of Hartmann sensors to wavefront metrology. Several applications are in areas that have historically been dominated by interferometric techniques. In particular, the use of interferometers to fabricate multi-meter size optics requires extensive metrology tooling costs that can exceed the costs to grind and polish the large optical components. The interferometric technology becomes increasingly difficult when one considers the fabrication and deployment of multi-segmented or membrane mirrors of the type that are now being considered for the Next Generation Space Telescope (NGST).
The adjustment of the six degrees of freedom of each segment of a multi-mirror usually requires several metrology schemes to cover the dynamic range. The initial departure from final figure shape may be several millimeters of inaccuracy, yet the final error must be as small as a few percent of the operational wavelength. This large dynamic range of measuring slope error should be possible in a single instrument so that an actuation system can be used to reduce the surface error to within a fraction of a wave of the desired shape.
Membrane mirrors have unique properties that mandate a new approach to measuring their surfaces. There are large amounts of 3rd and 5th order spherical aberration and the radius of curvature is also a variable. Membrane mirrors are very susceptible to acoustical disturbances that are exacerbated by the low mass, lack of damping and high stresses that give the mirror all of the characteristics of a drumhead.
For these applications, interferometers that measure optical path differences are inadequate. They need to operate near null, resulting in a low dynamic range to measure either surface figure or slope errors, cannot operate when the vibrational motion approaches the effective wavelength, and require considerable time to acquire sufficient data to give repeatable results.
Conventional Hartmann tests using an array of lenslets to sample the local tilts are much more applicable, since moderate amounts of vibration and turbulence can be averaged. The Hartmann system does not require a coherent source, and does not have the 2xcfx80 ambiguity of optical path difference associated with interferometric techniques. The problem with conventional Hartmann systems is that when one increases the dynamic range there is a commensurate decrease in accuracy and/or sampling density. The present invention presents a solution to all of the metrology applications in the above paragraphs while at the same time overcomes the limitations inherent in a conventional Hartmann instrument.
The present invention is of an adaptive dynamic wavefront sensor (and corresponding method) comprising: a spatial light modulator, and a lenslet array. In the preferred embodiment, a sub-array of pixels of the spatial light modulator controls illumination of a lenslet of the lenslet array. In one embodiment, the sub-array operates as a shutter for the lenslet, preferably selectively illuminating a subset of all lenslets of the lenslet array. This embodiment can be employed to increase the dynamic range of the sensor by allowing each lenslet focus to occupy a larger area of a detection device, to sample a wavefront at a variable density of points and frequencies to adaptively determine an optimal scan rate and scan configurations, or to adaptively change temporal frequency to quantify vibration amplitudes and modes. In another embodiment, the sub-array operates to control intensity of a focus of the lenslet. This embodiment can be employed to improve signal-to-noise ration, to change an effective f-number of the lenslet, or to apodize illumination of the lenslet to control aberration content of the beam from the lenslet.
The invention is also of an adaptive dynamic wavefront sensor (and corresponding method) comprising: a polarizer; pupil relay lenses; a spatial light modulator; a lenslet array; a CCD camera receiving light from the lenslet array; and a polarizing beam splitter receiving incoming light from the polarizer on one side and from the spatial light modulator on another side and sending light to the spatial light modulator on one side and to the lenslet array through the pupil relay lenses on another side.
A primary object of the present invention is provide an adaptive dynamic range wavefront sensor and method that can Individually control the apodization and intensity of light incident on a lenslet array of a Hartmann sensor.
A primary advantage of the present invention Is that it can be employed to simultaneously shutter a plurality of lenslets of the array or to selectively control illumination intensity and apodizaton of one lenslet or a plurality of lenslets.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.